A microwave oven heats food, but in the lab, microwaves have done something different: they turned a superconductor into a permanent magnet. A 300-nanometer-thick film of yttrium barium copper oxide, cooled to 80 kelvin, responded to 5-gigahertz radiation. This is the first observation of the stationary inverse Faraday effect in a high-temperature superconductor.

The essence of the effect is simple: light with circular polarization, meaning an electric field that rotates, makes electrons move in circles. In a normal conductor, these circular currents quickly die out, but in a superconductor they encounter no resistance. A permanent magnetic field arises, as if an invisible electromagnet had appeared inside the material. The key difference from ordinary coils is the absence of wires and heat losses.

As power increased, the signal grew until the calculated internal field exceeded 0.82 micronewtons per meter, the lower critical field at which vortices of magnetic flux begin to penetrate the superconductor. At that point, the signal dropped sharply to zero, and at even higher power, narrow peaks appeared at the third and fourth harmonics of the vortex lattice frequency. The material seemed to reconfigure, transitioning from a homogeneous state to an ordered vortex structure.

The practical value is obvious. In superconducting quantum circuits, magnetic biasing is currently done with inductive lines, which heat up and cause crosstalk. Microwave control of magnetization could replace these lines and reduce qubit decoherence. Moreover, the method is suitable for spectroscopy of vortex dynamics in type-II superconductors.

The microwave oven in the kitchen heats soup, but in a physics lab, it spins an electronic "gyroscope" without friction. The superconductor, like a frictionless top, keeps spinning after the external torque is removed, but now in the form of a permanent magnetic field. Perhaps soon such fields will control quantum computers contactlessly and without losses.